Compressor fouling is a significant operational problem faced by gas turbine operators. It negatively affects the overall gas turbine performance in terms of power output and heat rate. In order to recover gas turbine performance, operators use compressor washing. When the gas turbine cannot be stopped, on-line compressor washing is used. This technology utilizes water spray injection in front of the compressor inlet at full rotational speed. To provide good cleaning, water spray nozzles should be properly positioned and directed in order to allow uniform wetting of the compressor blades. At the same time, water droplets should not be collected on the air duct walls upstream of the compressor. Additionally, the water droplets should be big enough to be able to clean the compressor and, at the same time, small enough to avoid erosion of the blades. In order to provide gas turbine operators with guidelines for the proper selection and use of on-line washing systems, a numerical study has been carried out, based on typical washing systems installed on large industrial gas turbines. The study includes air flow analysis inside the air duct upstream of the compressor and calculations of the motion of water droplets injected into the air flow at different locations. The influence of the following parameters has been studied: droplet size, both injection direction and velocity, spray cone angle and location of the spray nozzles. “Fluent” CFD software was used. Effectiveness of the washing systems was evaluated by: a) amount of droplets entering the compressor (as mass percent of total injected water), b) uniformity of droplet mass distribution along the compressor first blade length and c) droplet accumulation on the surrounding surfaces (this being a negative phenomenon). The results showed that the effectiveness of the washing systems depends highly on the spray nozzle location (a total of six locations have been analyzed) and in each location — on droplet size and initial velocity. Special diagrams have been developed to illustrate the required optimal combinations of droplet size and initial velocity in each location of the spray nozzles. The general conclusion was that the proper design of any on-line washing system should be based on detailed CFD analysis of the motion of the injected droplets. The results could be helpful for gas turbine operators as well as for designers of the on-line washing systems.
Water fogging of inlet air has become widely acceptable for gas turbine power augmentation. Fogging systems are generally designed for a specific set of conditions called the design point. As a rule, the design point is specified to provide the maximum capacity of the fogging system at extreme ambient temperatures, when gas turbine power output is maximally decreased. Since the extreme ambient conditions rarely occur, the fogging system primarily operates at partial loads, when only a part of the installed cooling stages is in operation. At partial loads the fogging system does not provide uniform fogging of the inlet air, which could result in insufficient fog evaporation, less air-cooling and entrainment of non-evaporated water into the compressor. Prolonged operation at such off-design conditions requires careful tuning of the system in order to maximize cooling efficiency and eliminate a possible impact on gas turbine maintenance. This requires clear understanding of conceptual features of thin droplets evaporation and fog cloud behavior inside gas turbine inlet ducts. To this purpose, computational fluid dynamics (CFD) analysis was used. A CFD model is described, which comprises a straight duct equipped with fog nozzles operating in conditions similar to field conditions at design point, as well as, at partial load operation of the fogging system. The study focused on problems, which are critical in design and operation of real fogging systems, namely: single droplet, mono- and poly-fraction fog evaporation; influence of flow turbulent intensity; fog cloud shape and dimensions; polyfraction fog evaporation in the wide range of ambient conditions; over-spray and under-spray operation of the fogging system. The study results were helpful in the tuning of the installed fogging systems to site specific ambient conditions that provided efficient cooling with safe compressor operation. These results should be useful to both designers and operators of the fogging systems.
Water fogging of inlet air has become widely acceptable for gas turbine power augmentation. Fogging systems are generally designed for a specific set of conditions called the design point. As a rule, the design point is specified to provide the maximum capacity of the fogging system at extreme ambient temperatures, when gas turbine power output is maximally decreased. Since the extreme ambient conditions rarely occur, the fogging system primarily operates at partial loads, when only a part of the installed cooling stages is in operation. At partial loads the fogging system does not provide uniform fogging of the inlet air, which could result in insufficient fog evaporation, less air-cooling and entrainment of non-evaporated water into the compressor. Prolonged operation at such off-design conditions requires careful tuning of the system in order to maximize cooling efficiency and eliminate a possible impact on gas turbine maintenance. This requires clear understanding of conceptual features of thin droplets evaporation and fog cloud behavior inside gas turbine inlet ducts. To this purpose, computational fluid dynamics (CFD) analysis was used. A CFD model is described, which comprises a straight duct equipped with fog nozzles operating in conditions similar to field conditions at design point, as well as, at partial load operation of the fogging system. The study focused on problems, which are critical in design and operation of real fogging systems, namely: single droplet, mono- and poly-fraction fog evaporation; influence of flow turbulent intensity; fog cloud shape and dimensions; polyfraction fog evaporation in the wide range of ambient conditions; over-spray and under-spray operation of the fogging system. The study results were helpful in the tuning of the installed fogging systems to site specific ambient conditions that provided efficient cooling with safe compressor operation. These results should be useful to both designers and operators of the fogging systems.
High-pressure water fogging is a relatively new technique for gas turbine inlet air cooling. Nevertheless, up to now, several hundreds of fogging systems have been installed around the world and this figure is rapidly growing. A large number of the fogging installations provided sufficient experience to establish a general approach to design, operation and maintenance of such systems. However, a fogging system could require some tuning to a specific gas turbine configuration and site conditions. Israel Electric Corporation (IEC) has implemented fourteen fogging systems on 120–150 MW gas turbines, and each system was tuned to provide higher effectiveness. Several systems were basically modified in order to reduce the risk of compressor blade erosion. Subsequent field tests had shown that the goals of the system modification and tuning were successfully achieved. This allowed extending operational hours of the fogging systems and provided noticeable fuel savings. Moreover, during the summer 2005 the fogging systems helped to achieve the all-time high peak of electricity demand. Extended use of the fogging systems features a prolonged operation at partial cooling capacity and at varying ambient temperature, pressure and humidity. This required a thorough evaluation of the fogging systems performance, in order to provide the systems settings that would allow the most effective operation with minimal risk of damage to gas turbine components, mainly the compressor blades. The paper describes an approach and results of the fogging system performance evaluation at the different operational conditions. The method allows for the rate of compressor airflow as a function of the injected water flow rate as well as of ambient pressure, temperature and humidity. Actual operational limitations are also considered. The results are illustrated with the examples that correspond to real fogging systems operating under Israeli weather conditions. The method is general and, therefore, is applicable to other weather conditions and for different fogging systems.
Inlet air cooling by water fogging became very popular in recent years, because it is relatively simple and inexpensive technique for gas turbine power augmentation. Large experience established general practices of design, operation and maintenance of the fog systems. Nevertheless, fog systems could require some tuning to a specific gas turbine configuration and site conditions. Israel Electric Corporation (IEC) has implemented eight fog systems on 120–150 MW gas turbines, and each system was initially tuned to provide higher effectiveness. During hot weather conditions, noticeable additional power was obtained, which was very helpful to meet peak electricity demand. Based on these positive results, an additional six fog systems were installed on Frame 9E gas turbines. Initial operation of new fog systems revealed some unexpected problems. IEC undertook a thorough study of the problems including computational fluid dynamics (CFD) analysis of the inlet duct air flow. On the basis of the study results, significant modifications of the fog systems were carried out. Field tests of the modified systems showed that all goals of the modifications were successfully achieved.
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